Drupal 7 Module Development

Creating Your First Module
When Drupal encounters an event for which there is a hook (and there are
hundreds of such events), Drupal will look through all of the modules for matching
hook implementations. It will then execute each hook implementation, one after
another. Once all hook implementations have been executed, Drupal will continue
its processing.
In the past, all Drupal hook implementations had to reside in the .module file.
Drupal 7's requirements are more lenient, but in most moderately sized modules,
it is still preferable to store most hook implementations in the .module file.
Later in this book you will encounter cases where hook
implementations belong in other files. In such cases, the reasons
for organizing the module in such a way will be explained.
To begin, we will create a simple .module file that contains a single hook
implementation – one that provides help information.